Modifies the raise style for Python 3 support

The upstream kombu Travis testrunner had syntax errors for Python 3 interpreters. I've used a check for Python 3 that I found in other Kombu code. In Python 3, the raise syntax to have an exception use a different traceback is done using the [``with_traceback()``](https://docs.python.org/3.5/library/exceptions.html#BaseException.with_traceback) method.